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In 1962, NASA geophysicist Gordon J. F. MacDonald stated, “If the astronomical data are reduced, it is found that the data require that the interior of the Moon be less dense than the outer parts. Indeed, it would seem that the Moon is more like a hollow than a homogeneous sphere.”[7]

Physical chemist Harold Urey suggested the presence of negative gravitational anomalies where "there is either matter much less dense than the rest of the Moon, or simply a cavity."[7]

Between 1972 and 1977, lunar seismometers were installed on the Moon, by the Apollo missions, to record moonquakes. The Moon was described as "ringing like a bell" during some of those quakes, specifically the shallow ones.[8] This phrase was brought to popular attention in March 1970,[1] in an article in Popular Science.[9] When Apollo 12 deliberately crashed the Ascent Stage of its Lunar Module onto the Moon’s surface, it was claimed that the Moon rang like a bell for an hour, leading to arguments that it must be hollow like a bell.[1] Lunar seismology experiments since then have shown that the lunar body has shallow moonquakes that act differently from quakes on Earth, due to differences in texture, type and density of the planetary strata. However, according to Mainstream science, there is no evidence of any large empty space inside the Lunar body.[8]

Cornell University's Ask an Astronomer, run by volunteers in the Astronomy Department,[12] answered the question "Can we prove that the Moon isn't hollow?". Physicist Suniti Karunatillake suggests that there are at least two ways to determine the distribution of mass within a body. One involves moment of inertia parameters, the other involves seismic observations. In the case of the former, Karunatillake points out that the moment of inertia parameters indicate that the core of the moon is both dense and small, with the rest of the moon consisting of material with nearly-constant density. As for the latter, he notes that the moon is the only planetary body besides Earth on which extensive seismic observations have been made. These observations have constrained the thickness of the moon's crust, mantle and core, suggesting it could not be hollow.[13]

Support for a solid lunar interior is based on:

Seismic observations. Besides Earth, the Moon is the only planetary body with a seismic observation network in place. Analysis of lunar seismic data have helped constrain the thickness of the crust (~45 km)[11][14] and mantle, as well as the core radius (~330 km).[10]

Moment of inertia parameters. For the Moon, moment of inertia parameters have demonstrated that the core is ~1.4% of the total mass.[15] One such parameter, the normalized polar moment of inertia, is 0.393 ± 0.001.[15][16] This is very close to the value for a solid object with radially constant density, which would be 0.4 (for comparison, Earth's value is 0.33).[15] The normalized polar moment of inertia for a hollow Moon would have a higher value, closer to 0.67.[17]